This invention relates to improved electrically conductive, filament-reinforced plastic articles, such as plastic sheets and pipe.
Plastic pipe has been used for many years to provide corrosion-proof conduits in many different types of industries. A disadvantage of prior art plastic pipe is that it can develop a dangerous electrostatic charge because of its low electrical conductivity. For example, Lloyd's of London requires that all fiberglass pipe used to handle petroleum products on board ship have a surface resistivity of less than 10.sup.9 ohms (when tested in accordance with ASTM D257-76) so that static charges built up in the pipe by fluid flow may be conducted to ground. Conventional fiberglass-reinforced pipe made before this invention has a surface resistivity of about 10.sup.12 ohms, and therefore is unsatisfactory for such applications. Economically priced, electrically conductive plastic sheets would also find wide use to shield against radio frequency electromagnetic radiation.
Before this invention, conductive materials, such as graphite, carbon, silver, aluminum, and copper powder and particles, have been incorporated into rubber and plastic products to make them more conductive to prevent static charge buildup, or to make radio frequency shields. Chopped metallic fibers and chopped conductive glass filaments have also been used in plastic products to increase electrical conductivity. The article entitled "Conductive Polymeric Compositions" in Polymer Engineering and Science, December 1977, Vol. 17, No. 12, discloses the use of conductive fibers, spheres, flakes, and irregular particulates of conductive materials to increase the thermal and electrical conductivity of plastic articles. The article states that fibrous conductors improve conductivity more significantly than spheres, flakes, or irregular particles. However, the article limits its disclosure to particles and short glass fibers having a maximum aspect ratio of no more than approximately 35:1. The aspect ratio is the ratio of the length of a conductive member to its diameter. Moreover, the use of conductive powder, particles, and chopped filaments is expensive, because relatively large amounts of these materials are required to achieve the necessary modification of electrical conductivity.
Thus, although the advantages of electrically conductive plastic have long been known, their use has been limited in many applications, because they have not been economically feasible before this invention.
In making conventional filament-reinforced plastic pipe, hundreds of filaments of reinforcing material, such as fiberglass, nylon, etc., are combined to form an "end" or roving. About 15 ends are combined to form a "strand", which includes several thousand individual filaments; and between about ten and about 40 strands are combined to form a tape, web, or band, which is saturated with liquid resin to coat the individual filaments with plastic. The resin-saturated band is wound onto a rotating mandrel. The winding path is usually a simple helix at an angle of about 54.degree. to the mandrel axis of rotation. A uniformly reinforced pipe wall is formed by winding the band in overlapping helices, first to the right along the mandrel and then to the left. The filaments are substantially continuous, unidirectional fibers, and may be mineral fibers, such as glass or asbestos; animal fibers, such as wool; vegetable fibers, such as cotton; or synthetic fibers, such as nylon, rayon, dacron, orlon, and the like.
The resin or adhesive used to impregnate the band and wet the filaments can be any suitable thermosetting or thermoplastic resin used in winding or laminating procedures. For example, binders containing a thermosetting resin, such as epoxy, polyester, vinyl ester, furan, phenol-formaldehyde, or the like, or binders containing a thermoplastic resin, such as polyvinyl chloride, polyvinylidene chloride, or the like can be used.
U.S. Pat. No. 3,499,815 to Hof, and U.S. Pat. No. 3,519,520 to Newman describe apparatus and methods for making conventional fiber-reinforced plastic pipe. Such pipe is satisfactory under many conditions where resistance to corrosion is required, but it is of limited use where buildup of electrostatic charge creates a hazardous condition.
U.S. Pat. No. 3,070,132 to Sheridan; U.S. Pat. No. 3,555,170 to Petzetakis; U.S. Pat. No. 3,580,983 to Jackson; U.S. Pat. No. 3,958,066 to Imamura et al; and U.S. Pat. No. 3,963,856 to Carlson et al describe various conventional ways of increasing the thermal and electrical conductivity of plastic products by using metal wires or particles of metal or carbon imbedded in the product. As indicated above, this technique is expensive and has not been widely used to produce plastic pipes, for example, on a commercial basis.